Proper date formatting of string read from a SqlDataReader

You don't want to convert the object that is returned from the indexer to a string, before you hand it off to a String.Format. By leaving out the ToString call you would have got what you want.

lblTDate.Text = string.Format("{0:D}", DT2["TDate"]);

This enables the String.Format implementation to use IFormattable implementation of the DateTime type which makes that the format string you've found will work. Once it is a string there is no way for the String.Format implementation to know what to do.

Alternatively cast the value to the IFormattable interface, like so:

var fmt = DT2["creationdate"] as IFormattable;
if (fmt != null)
{
    lblTDate.Text = fmt.ToString(
        "dddd, MMMM dd, yyyy",
        System.Globalization.CultureInfo.InvariantCulture);
}

but then you have to provide an IFormatProvider implementation, often in the form of an instance of a CultureInfo class.

Notice that in both of the above options, you don't have to to convert the value to a DateTime first.

CultureInfo culture = new CultureInfo("en-Us");
DateTimeFormatInfo dtfi = culture.DateTimeFormat;

var myDate = new DateTime(2016, 7, 4, 12, 0, 0);

string month = culture.TextInfo.ToTitleCase(dtfi.GetMonthName(myDate.Month));
string dayOfWeek = culture.TextInfo.ToTitleCase(dtfi.GetDayName(myDate.DayOfWeek));
string fullDate = $"{dayOfWeek}, {month} {myDate.Day}, {myDate.Year}";

Console.Write(fullDate);